1. Field of the Invention
Aspects of the invention relate to switching power supplies exhibiting improved efficiency in a light load condition.
2. Description of the Related Art
To ensure stability and safety of commercial power systems, power factor correction is obligated to switching power supplies with a power consumption larger than 75 W. Accordingly, proposed recently are switching power supplies composed of a power factor correction converter (PFC) with a small sized and high efficiency and a DC-DC converter that converts a DC voltage obtained using the power factor correction converter to a DC output voltage corresponding to a specification of a load. Japanese Unexamined Patent Application Publication No. 2007-288855 (also referred to herein as “Patent Document 1”), for example, discloses this type of switching power supply. Most of such DC-DC converters, with a rated load power of about 100 W, employ a quasi-resonance (QR) converter, which impose relatively little burden on a secondary side rectifying diode.
FIG. 8 shows a schematic construction of a switching power supply 1 comprising a power factor correction converter 2 and a DC-DC converter 3, which is a quasi-resonance converter. A rectifying circuit 4 rectifies an AC power supplied from a commercial power supply 5 through a noise filter 6 and delivers to the power factor correction converter 2.
The power factor correction converter 2 is basically composed of an inductor L1 connected to the rectifying circuit 4 and a switching element Q1 to form a current path through the inductor L1 in an ON period of the switching element Q1. The power factor correction circuit 2 also comprises a diode D1 to form a current path between the inductor L1 and an output capacitor C2 in an OFF period of the switching element Q1. The control circuit IC1 ON/OFF-drives the switching element Q1 and controls the current through the inductor L1 to obtain a stabilized DC voltage Vb.
Resistors R1 and R2 divide the DC voltage Vb obtained across the output capacitor C2 to detect the voltage Vb, and feeds back the detected voltage to the control circuit IC1. A shunt resistor R3 detects the current flowing through the inductor L1. Japanese Unexamined Patent Application Publication No. 2010-220330 (also referred to herein as “Patent Document 2”), for example, discloses operation and effect of such a power factor correction converter 2 in detail.
The DC-DC converter 3, which is a quasi-resonance converter, is basically provided with a switching element Q2 connected in series to a primary winding P1 of an isolation transformer T, the primary winding P1 receiving the output, the DC voltage Vb, of the power factor conversion converter 2. The DC-DC converter 3 is also provided with a resonance capacitor C4 in parallel with the switching element Q2 and an output capacitor C5 connected through a rectifying diode D2 to the secondary winding S1 of the isolation transformer. A control circuit IC2 ON/OFF-drives the switching element Q2 to generate a quasi-resonant oscillation in a resonance circuit composed of a leakage inductance of the isolation transformer T and the resonance capacitor C4, thereby generating a specified DC output voltage Vo.
Resistors R4 and R5 divides the DC output voltage Vo obtained across the output capacitor C5 to detect the output voltage Vo and feeds back the divided voltage to the control circuit IC2 through a feedback circuit FB. A shunt resistor R6 detects the current flowing in the switching element Q2. The DC-DC converter 3 detects a ZCD voltage developed across an auxiliary winding P2 of the isolation transformer T and controls the turning ON timing of the switching element Q2. Japanese Unexamined Patent Application Publication No. 2011-015570 (also referred to herein is “Patent document 3”), for example, discloses details about operation and effect of such a DC-DC converter 3, which is a quasi-resonance converter.
The switching power supply 1 significantly improves power factor thereof owing to the power factor correction converter 2 provided on the preliminary stage of the DC-DC converter 3. The power factor correction converter 2 however, also generates energy loss inevitably. Especially in a light load condition, a switching frequency becomes high in both the power factor correction converter 2 and the DC-DCC converter 3. Therefore, switching loss increases in the switching elements Q1 and Q2 deteriorating the efficiency of the switching power supply 1.
In order to reduce the switching loss in the switching elements Q1 and Q2, International Patent Application Publication No. WO2004/023634 (also referred to herein as “Patent Document 4”), for example, discloses a control method of so-called bottom skip which uses a timing at which a resonant oscillation current that arises after turning OFF of the switching elements Q1 and Q2 becomes zero. This bottom skip control delays a turning ON timing of the switching elements Q1 and Q2 in a light load condition to reduce a switching frequency, thereby restraining a loss. The number of bottom skips in the bottom skip control is set at [0] in a normal condition, or heavy load condition, and set at gradually larger values as the load becomes lighter.
The bottom skip control is conducted in the power factor correction converter 2 and DC-DC converter 3 by detecting a load condition, a magnitude of the load, with a load detecting means provided in the control circuits IC1 and IC2. Conducting such a bottom skip control, however, does not necessarily improve efficiency. In a heavy load condition, for example, conduction loss is generally dominant over switching loss. As a result, switching frequency reduction in a heavy load condition increases the conduction loss, rather deteriorating the efficacy.
Consequently, a load condition, or a magnitude of the load, needs to be detected precisely in order to obtain optimum efficiency. A load condition detecting means in a DC-DC converter 3 generally carries out load condition detection based on information about the DC output voltage Vo obtained through the feedback circuit FB. Here, an input voltage to the DC-DC converter 3 is stabilized by the power factor correction converter 2. Consequently, the load condition is detected precisely in the DC-DC converter 3.
On the other hand, a load a condition detecting means in the power factor correction converter 2 detects a load condition from an information about the load current detected through the shunt resistor R3. Here in the power factor correction converter 2, a magnitude of an inductor current is controlled corresponding to the phase angle of the input AC voltage Vac. As a consequence, the detection precision of the load condition in the power factor correction converter 2 changes inevitably depending on the phase angle of the input AC voltage Vac. It is therefore difficult to detect the load condition with high precision in the overall input voltage range of the input AC voltage Vac.